Heparin-induced thrombocytopenia (HIT) is an iatrogenic cause of thrombocytopenia and thrombosis due to antibodies to complexes between Platelet Factor 4 (PF4) and heparin. Around half of heparinized patients develop such antibodies after cardiopulmonary bypass (CPB), yet only 1-5% of them develop HIT. What is the molecular basis of this observation? We have recently made two important findings that we believe enhance our understanding the pathophysiology of HIT and explain this clinical observation. Over a narrow molar range of reactants: (1) PF4 and heparin form soluble antigenic ultralarge complexes (ULCs) and (2) PF4 and platelet surface glycosaminoglycans (GAGs) form antigenic complexes, which are key to the development of thrombocytopenia in a murine model of HIT. We will now further develop this model and study its relationship to HIT through the following specific aims: Aim 1: In vitro characterization of PF4/GAG surface complexes. Surface GAGs on macrophages and endothelial cells differ from those on platelets and their composition is modified by inflammation. We will define the biological features of antigenic PF4/GAG complexes on these cells to help explain why HIT is so frequently complicated by thrombosis. We will characterize the effect of heparin on their composition and antigenicity, and define the role of soluble PF4/heparin complexes in the development of HIT. Aim 2: In vivo studies of surface PF4/GAG in HIT. In vivo studies supporting the central role of surface PF4/GAG have been performed using a murine model of HIT with a HIT-like monoclonal antibody KKO. We will now extend these studies using HIT IgG. We have also established a HIT model with thrombosis based on an arterial injury and will define the role of surface PF4/GAGs in thrombotic complications. We will also examine the role of atherosclerotic vascular damage in our model, as most HIT patients have either underlying atherosclerosis and/or vascular inflammation. Finally, we will pursue novel therapeutic interventions in our murine HIT models based on disruption of antigenic PF4/GAG complexes. Aim 3: Measurement of platelet surface PF4 in clinical samples. We will pursue the hypothesis that a subset of the population have high platelet PF4 content and are thereby at highest risk to form antigenic PF4/GAG complexes when their platelets are activated, e.g., by atherosclerosis, and they are exposed to heparin. We will measure total platelet PF4, surface PF4 and antigenic PF4/GAG complexes in a well pediatric population compared with an adult population pre-CPB. These studies should support our in vitro and murine in vivo models and set the stage for a large prospective analysis of the predictive value of measuring platelet-PF4 as a risk factor for HIT. Taken together, our studies will improve our understanding of the pathophysiology of HIT, may allow pre- selection of patients at high risk to develop the disease and will provide a platform to develop novel therapeutic interventions that disrupt the pathogenesisof HIT.